Coherent detection of optical signals is once again of interest for applications in fiber optic communications, due to increasing demand for higher throughput, optical communication systems use advanced modulation formats which require increasing spectral efficiency of the system, such as differential QPSK (DQPSK), in which information bits are coded as phase transient between adjacent symbols. However, the signal is more and more sensitive to link impairment, such as dispersion and Polarization Mode Dispersion (PMD), which introduces amplitude and phase distortion to the optical signal. Therefore, efficient recovery should include not only signal amplitude, but also phase of the received optical signal.
In the coherent detection technique, an important aspect is Carrier Phase Recovery (CPR) or estimation (which can be made at the transmitter or at the receiver) which recovers and compensates for phase noise in the received optical signal (which originates from phase noise of the transmitting laser source or of the local oscillator of the receiver), thus enabling recovery of the information data.
Conventional methods for demodulating phase-shift keying (PSK) signals, which are based on digital carrier-phase estimation, are described for example, in “Digital Equalization of Chromatic Dispersion and Polarization Mode Dispersion,” J. M. Kahn, Journal of Lightwave Technology, vol. 25, No. 8, pp. 2033-2043, August 2007, as well as in “Phase Estimation Methods for Optical Coherent Detection Using Digital Signal Processing,” M. G. Taylor Journal of Lightwave Technology, vol. 27, no. 7, pp. 901-914, April 2009.
Carrier-phase estimation can restore the in-phase and quadrature components of the complex amplitude of the electric field the optical signal. However, the obtained complex amplitude includes phase-noise, which stems from semiconductor lasers used for the transmitter and local oscillator, which decreases the performance of the receiver.
“Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” Gagnon et al, Journal of Lightwave Technology, vol. 24, no. 1, pp. 12-21, January 2006 discloses a conventional Mth power scheme to raise the received MPSK signals to the Mth power to estimate the phase reference in conjunction with a coherent optical receiver. By raising the received MPSK signals (which are complex signals) to the Mth power, the data carried by the complex signal is isolated from the phase of the signal and therefore, it is easier to estimate the phase. However, this scheme requires nonlinear operations, such as Mth power and arctan(·) which introduces a large latency to the detection system.
“Unrepeated 200-km transmission of 40-Gbit/s 16-QAM signals using digital coherent receiver,” to Y. Mori et al (Opt. Exp., vol. 17, no. 3, pp. 1435-1441, February 2009) discloses another carrier-phase estimation method, which is based on the decision-feedback loop and the Least Mean Square (LMS) algorithm of the complex field amplitude. However, this method requires complex multiplication and sample-wise adaptation of the tap coefficient.
It is therefore an object of the present invention to provide a method for efficiently estimating the phase of a modulated optical signal, while reducing the Additive White Gaussian Noise.
It is another object of the present invention to provide a method for efficiently estimating the phase of a modulated optical signal, while reducing the complexity of the required hardware.
Other objects and advantages of the invention will become apparent as the description proceeds.